In the electric motor industry, it has become increasingly popular to use components including high frequency power converters (e.g., pulse width modulators) to control electric motors, such as motor speed, torque, velocity, acceleration, etc. However, an unintended consequence of the use of components such as high frequency power converters is the creation of capacitive-coupled electric currents, sometimes referred to as leakage current. This leakage current can conduct through the primary insulation of an electric motor, through the motor components including the ball bearings, and into the electrical ground of the motor. The leakage current can eventually cause the raceways and roller balls of the bearings to mechanically break down and fail in a premature manner.
Various structures for reducing leakage current through electric motors are known. These structures are provided in an attempt to reduce leakage current flowing through the internal components of the electric motor, such as the bearings. For example, Japanese Reference Nos. 63-161835, 2055546, 01-231633, and 4-117148 all disclose structures provided in attempts to reduce leakage current through internal components of electric motors. However, the structures disclosed in these references are complex in design and can be difficult to incorporate into the manufacturing process of the electric motors.
For example, structures are disclosed in Japanese Reference No. 4-117148 for protecting a bearing from electrolytic corrosion. These structures include insulators with apertures to accommodate fasteners extending through the insulators to couple various components of the motor together. However, structures that include apertures for fasteners can be difficult to handle during the manufacturing process of the motor since such apertures must be aligned with other apertures provided on components of the motor to affix the structures in place. In addition, undesirable conduction paths for the leakage current can be provided by the fasteners running through the structures.
Other embodiments of structures meant to reduce the conduction of leakage current are disclosed in Japanese Reference No. 4-117148 and are formed of multiple pieces which together are configured to reduce leakage current. However, structures formed of multiple pieces can also be difficult to assemble during manufacture of the motor.
Other designs have also been used in an attempt to reduce the effects of leakage current. For example, conductive grease has been used to surround the balls of a bearing of a motor to conduct leakage current around or away from the balls. In other designs, portions of bearings have been made of non-conductive ceramic material, or additional brushes have been configured to contact the shaft of the motor and thereby shunt leakage current around the bearings. In yet other designs, plastic “cups” have been used to surround the bearing in an attempt to increase the impedance for leakage current traveling to the bearings. However, these designs can be undesirable in that they can be difficult to implement, require multiple components made of various non-conductive material, may not provide adequate protection against leakage current, and can have other undesirable consequences such as increasing the working temperature in the bearings.
Accordingly, while structures for reducing leakage currents through an electric motor are known, it is desirable to provide members that are simple structurally, provide adequate reduction in leakage current through internal components of electric motors, and can be easily incorporated into the manufacturing process of the electric motors.